The molecular graphics system has been used to determine models for the structure of a number of biologically important macromolecules. The modeling starts with the high resolution structure of one protein determined by crystallographic techniques. The amino acid side chains of a related protein are graphically substituted where they differ. Additions and/or deletions in the poly peptide chain are made and modeling is done to make the chain contiguous and stereochemically correct. The technique has been applied to immunoglobulins, myoglobins, insulins, thrombin, factor IXa, factor Xa, calmodulin and interferon. Success with the technique varies with the degree of similarity between the starting and modeled structures. Space filling images in color and stereo made on the molecular display system of the modeled structures provides a departure point for thinking and biochemical experimentation.